Light is a transverse electromagnetic wave meaning that the electric and magnetic vectors are perpendicular to the direction of travel. Two important characteristics of light waves can be defined, and these are known as intensity and polarization. The square of the magnitude or amplitude of the electric vector is proportional to the intensity (or brightness of the light). The angular orientation of the electric vector about the propagation direction is characterized as its linear polarization direction. Light where the electric vector rotates about the direction of propagation is known as either circularly or elliptically polarized light. Polarizing devices (such as sheet Polaroid) are capable of selectively transmitting, absorbing or reflecting polarized light with a given linear polarization direction. Both monochromatic natural light (filtered to yield a single color) and laser light can be used in devices known as interferometers which are used to measure displacement or distance. Lasers are best suited for such distance measurements when the distances involved exceed 1 cm.
The science of interferometry involves the splitting of light waves, their propagation over different geometrical paths and the study of the optical phase and intensity relationships that occur as a result of these path differences when these light waves are recombined.
Special optical elements known as beamsplitters can also be used to amplitude divide light waves. When these devices are implemented, they create two light waves which each contain one half of the total amount of light originally present in the incident wave. One of the two waves serves as a reference wave, whereas the other serves as the measuring or sensing means. If the electric vectors of the two waves are in phase (both vectors have the same sign) then reinforcement occurs. If the two waves are out of phase (one vector positive and the other negative) then cancellation occurs. If one of the two waves travels a fixed or reference distance and the other travels a variable distance, then the phase of the second wave will change with a change in distance. The interfering waves alternatively cancel and reinforce depending on their phase differences, and this phenomena is used for precise measurement of distance by counting the fringes.
One important class of amplitude division interferometry takes advantage of the polarization properties of the light waves.
The use of polarizing devices makes it possible to observe specific phase difference conditions between the two light waves. Two separate phase conditions need to be observed to uniquely obtain distance and direction of motion relative to a time reference and it was common to use the condition where the phases of the two waves differed by 90 degrees. This method derived from early encoder technology which was an optomechanical technique used for precise distance measurement prior to the invention of the laser. These devices were used for machine control applications.
With the advent of lasers, the 90 degree phase shift corresponds to a distance change of one eighth of a wavelength. With one signal as a reference, the second signal will lead or lag it by this phase shift, therefore providing the necessary direction sensing information. These two signals are known to be in phase quadrature.
When two light waves are divided and then recombined to produce interference fringes, the resultant signal undergoes a sinusoidal modulation and the average value of the signal is offset relative to zero light intensity. Such light signals can be said to consist of an AC or alternating or modulating term with the offset know as a DC term. Furthermore, if the measuring light wave is partially blocked as can be the case in machine control applications where metal chips or cutting fluids interfere with the beam, the peak to valley modulation is reduced. The AC signal does not go to zero and the DC offset term can drift around. The reduction in the AC modulation is generally not serious, but the DC drift can lead to situations where false fringe counts maybe generated when the signals are recombined. It is possible to eliminate the DC term by selecting a third phase condition where the two beams are 180 degrees out of phase. Subtracting the 90 degrees from the 180 degrees out of phase signals and from the in phase signal produces two new signals which are themselves 90 degrees out of phase and independent of the DC term. The prior art discloses many examples by which the three phase conditions can be produced with interfering light waves. These early interferometer systems can all be divided into two subsystems.
One subsystem concerns the amplitude beamsplitting means, and the other subsystem is the means used to select or decode the two or more phase conditions required for signal processing. The amplitude beamsplitting means can be classified as either the Michelson type or the polarizing type. In the Michelson type, light of different polarization conditions can be both transmitted and reflected. In the polarizing type, light of one preferred polarization direction is transmitted while a second polarization direction, 90 degrees to the first, is reflected. Downs (U.S. Pat. No. 4,360,271), Erickson (U.S. Pat. No. 3,601,490) and Hock (U.S. Pat. Nos. 3,529,894 and 3,822,942) are illustrative of the use some form of Michelson type splitting. Russo (U.S. Pat. No. 3,771,875) uses a combination of both types of splitting. Morokuma (U.S. Pat. No. 3,976,379) and Lacombat (W. German Pat. No. 2,111,936) use polarizing type beamsplitting.
Prior art interferometers use a variety of different techniques in the second subsystem in order to decode the different phase conditions. The prior art decoding devices are generally very complex mechanically requiring several beamsplitters, polarizing devices and other optical elements; They generally require special and in some cases very difficult and therefore costly evaporated coatings and the division of the light amplitudes with the decoder is not efficient because in some cases, light not used by the detection means is wasted so as to effectively preclude using a single laser source for multi axis distance measurement applications.